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931.
We present a theoretical study on the effects of key catalyst layer (CL) design parameters on the cold start behavior of a polymer electrolyte fuel cell (PEFC) using a three-dimensional transient cold start model developed in a previous study 1 and 2. Among several CL design parameters, we adopt the ionomer fraction (?I) and weight ratio of Pt to carbon support (wt%Pt–C) in the cathode CL as CL design variables for this study. Therefore, other design parameters such as CL thickness and the oxygen reduction reaction (ORR) kinetic parameter are accordingly adjusted due to changes in ?I and wt%Pt–C for cold start simulations. The calculated results confirm that these two design parameters provide control of the ice storage capacity and water absorption potential of the cathode CL, and consequently have a substantial influence on the cold start behavior of a PEFC. We provide a guideline to design and optimize a cathode CL and membrane electrode assembly (MEA) for improved PEFC cold start capability. 相似文献
932.
A novel micro-porous layer (MPL) is designed to enhance the cold-start capability of a polymer electrolyte fuel cell (PEFC). The concept of designing an MPL is to expand the ice storage capacity of the electrode into the MPL region. We impose proton conduction capability and the oxygen reduction reaction (ORR) kinetic activity on the MPL via controlling the platinum (Pt) loading, ionomer fraction and weight ratio of Pt to the carbon support (wt%Pt–C) of the MPL. Therefore, the MPL is dual-functional, and can work as a typical MPL for normal PEFC operations and as a part of the cathode catalyst layer (CL) for cold-start operations. Three-dimensional (3-D) cold-start simulations are carried out by using a 3-D cold-start model developed in a previous study [1]. The detailed simulation results clearly suggest that the cold-start operational time can be extended significantly using a dual-function MPL, and the extended time is directly proportional to the pore volume of the MPL for ice storage. This study provides a new strategy to enhance the cold-start capability of a PEFC by properly designing and optimizing the MPL. 相似文献
933.
Ashok R. Patel Patricia C.M. Heussen Johan HazekampEllen Drost Krassimir P. Velikov 《Food chemistry》2012
Quercetin loaded biopolymeric colloidal particles were prepared by precipitating quercetin (water insoluble polyphenol) and zein (hydrophobic protein), simultaneously, by adding their hydro-alcoholic solution to aqueous solution in presence of sodium caseinate as an electrosteric stabiliser. The presence of protein resulted in altering the shape of quercetin precipitates from needle-like to spherical shape at higher zein proportions, as confirmed by transmission electron microscopy. The average particle size of zein:quercetin composite particles was below 200 nm (130–161 nm) with negative surface charge (−30 to −41 mV), as confirmed by dynamic light scattering and electrophoretic mobility data. Solid state characterisation (X-ray diffraction) and spectroscopic measurements (UV–Vis and IR spectroscopy) confirmed characteristic changes in quercetin due to the entrapment in the biopolymeric matrix of colloidal particles. Results from anti-oxidant study demonstrated the advantage of entrapping quercetin in the colloidal particles in terms of the chemical stability in the alkaline pH and against photodegradation under UV-light irradiation. 相似文献
934.
The preparation of a yellow dextrin is thoroughly characterized during different stages of its production process with a SEC system equipped with four detectors (light scattering, viscosity, UV and RI). With this system, also referred to as SEC‐4D, it was shown that the changes in characteristics are only in line with initial expectations up to 5 h after initiation of the reaction. After 5 h, changes in molar mass, intrinsic viscosity and colour point to the presence of significantly different structures. The combined results of the SEC‐4D measurements indicate the formation of aggregate like structures made of small starch fragments which are intensely coloured and physically linked. The obtained results also indicate that the aggregate like structures are probably still susceptible to repolymerization and transglycosylation. The newly derived parameter YI‐400 nm played an important role in defining a plausible cause for the observed changes in characteristics. 相似文献
935.
Olivier Basl Johan Bidange Qi Shuai Chao‐Jun Li 《Advanced Synthesis \u0026amp; Catalysis》2010,352(7):1145-1149
An efficient method was developed for the direct acylation of arene sp2 C H bonds with aldehydes using palladium acetate as catalyst and peroxide as oxidant. The solvent‐free oxidative acylation reaction assisted by a pyridine directing group provides an easy access to aromatic, aliphatic, and optical active ketones. 相似文献
936.
Stefan Jarau Johan W. van Veen Robert Twele Christian Reichle Eduardo Herrera Gonzales Ingrid Aguilar Wittko Francke Manfred Ayasse 《Journal of chemical ecology》2010,36(6):565-569
Reproductive division of labor in advanced eusocial honey bees and stingless bees is based on the ability of totipotent female
larvae to develop into either workers or queens. In nearly all species, caste is determined by larval nutrition. However,
the mechanism that triggers queen development in Melipona bees is still unresolved. Several hypotheses have been proposed, ranging from the proximate (a genetic determination of caste
development) to the ultimate (a model in which larvae have complete control over their own caste fate). Here, we showed that
the addition of geraniol, the main compound in labial gland secretions of nurse workers, to the larval food significantly
increases the number of larvae that develop into queens. Interestingly, the proportion of queens in treated brood exactly
matched the value (25%) predicted by the two-locus, two-allele model of genetic queen determination, in which only females
that are heterozygous at both loci are capable of developing into queens. We conclude that labial gland secretions, added
to the food of some cells by nurse bees, trigger queen development, provided that the larvae are genetically predisposed towards
this developmental pathway. In Melipona beecheii, geraniol acts as a primer pheromone representing the first caste determination substance identified to date. 相似文献
937.
Marco W.M. van Goethem Simon Barendregt Johan Grievink Jacob A. Moulijn Peter J.T. Verheijen 《Chemical Engineering Research and Design》2010
The steam cracking practice seems to have reached a stage of maturity which makes it increasingly difficult to improve ethylene yield. In order to determine if there is still scope for yield improvements it is helpful to know what the optimal reaction conditions for the steam cracking process are. This work presents a model-based synthesis approach that enables to determine the optimal thermal and physical reaction conditions for a particular feed, maximising olefin yield. A distributive reaction-mixing synthesis model has been combined with an industrially proven large kinetic scheme, SPYRO®, which contains over 7000 reactions between 218 molecular and 27 radical species. The model combination allows optimising the following degrees of freedom with respect to olefin yield: feed distribution, product removal, macro-mixing, along a reaction volume coordinate. The reaction temperature upper limit is put at 1300 K, exceeding the current (metallurgical) bound by 100 K. For the cracking of ethane a linear-concave unconstrained temperature profile with a maximum temperature of ∼1260 K proves optimal which is lower than allowed while all ethane should be supplied at the entrance of the reaction volume. For propane and heavier feedstocks an isothermal profile at the upper temperature bound, with dips at the beginning and the middle of the reaction coordinate is optimal, while distribution of the hydrocarbon feed along the reactor coordinate results in higher yields. The theoretical maximum achievable ethylene yield for ethane cracking is found to be 66.8 wt% while in conventional cracking typically 55 wt% is considered to be the maximum value. This optimum is constrained by the pressure which is at its lower bound. The resulting residence time is in the same order as with current technology for ethane cracking. For the more heavy feedstocks these times are one order of magnitude smaller which will be a challenge for designing. 相似文献
938.
Vinit P. Chilekar John van der Schaaf Ben F. M. Kuster Johan T. Tinge Jaap C. Schouten 《American Institute of Chemical Engineers》2010,56(3):584-596
This article reports on the influence of elevated pressure and catalyst particle lyophobicity at particle concentrations up to 3 vol % on the hydrodynamics and the gas‐to‐liquid mass transfer in a slurry bubble column. The study was done with demineralized water (aqueous phase) and Isopar‐M oil (organic phase) slurries in a 0.15 m internal diameter bubble column operated at pressures ranging from 0.1 to 1.3 MPa. The overall gas hold‐up, the flow regime transition point, the average large bubble diameter, and the centerline liquid velocity were measured along with the gas–liquid mass transfer coefficient. The gas hold‐up and the flow regime transition point are not influenced by the presence of lyophilic particles. Lyophobic particles shift the regime transition to a higher gas velocity and cause foam formation. Increasing operating pressure significantly increases the gas hold‐up and the regime transition velocity, irrespective of the particle lyophobicity. The gas–liquid mass transfer coefficient is proportional to the gas hold‐up for all investigated slurries and is not affected by the particle lyophobicity, the particle concentration, and the operating pressure. A correlation is presented to estimate the gas–liquid mass transfer coefficient as a function of the measured gas hold‐up: $k_{\rm l}a_{\rm l}/\varepsilon_{\rm g} = 3.0 \sqrt{Du_{\rm b}/d_{\rm b}^3}\;{\rm s}^{-1}$ . © 2009 American Institute of Chemical Engineers AIChE J, 2010 相似文献
939.
Junhua Ji Christophe A. Codemard Jayanta K. Sahu Johan Nilsson 《Optical Fiber Technology》2010,16(6):428-441
In this paper, we present basic design rules for double-clad fibers that enable efficient 1st-Stokes operation of cladding-pumped fiber Raman devices. Limiting factors that we treat include unwanted 2nd-Stokes generation, material damage in the core, pump pulse dispersion, inner-cladding NA, background loss, pulse shape and pump noise. With a well-designed fiber, we calculate that a pump-to-signal brightness enhancement of over 2000 is possible, with certain pump parameters. On experimental work, we report a 100 W cladding-pumped fiber Raman laser at 1120 nm. Furthermore, a double-clad Raman fiber with large-mode area, ~40 μm core diameter, pumped by a Q-switched Nd:YAG laser is used to generate Stokes pulses with 1 mJ energy. 相似文献
940.
VJ Ruigrok M Levisson J Hekelaar H Smidt BW Dijkstra J van der Oost 《International journal of molecular sciences》2012,13(8):10537-10552
Aptamers are oligonucleotide ligands, either RNA or ssDNA, selected for high-affinity binding to molecular targets, such as small organic molecules, proteins or whole microorganisms. While reports of new aptamers are numerous, characterization of their specific interaction is often restricted to the affinity of binding (K(D)). Over the years, crystal structures of aptamer-protein complexes have only scarcely become available. Here we describe some relevant technical issues about the process of crystallizing aptamer-protein complexes and highlight some biochemical details on the molecular basis of selected aptamer-protein interactions. In addition, alternative experimental and computational approaches are discussed to study aptamer-protein interactions. 相似文献